(1) Field of the Invention
The present invention relates to a catalyst useful for purifying the harmful elements in the exhaust gas such as nitrogen oxide (NO.sub.x), carbon monoxide (CO) and hydrocarbon (HC), particularly one useful for purifying the harmful elements in the auto emission gas.
(2) Prior Art
The catalyst employed for purifying the exhaust gases emitted from internal combustion engines like the automobile has to function under specific conditions in which the factors largely affecting the velocity of chemical reaction such as the volume or concentration of the reactants, or working temperature cannot be stabilized; and accordingly it is required to exhibit a high catalytic activity over a very wide range of temperatures.
As such an exhaust gas purifying catalyst, a catalyst constituted of a specific catalytic element carried on a carrier has been known. For the purpose of eliminating the major harmful elements in the auto exhaust gas, i.e., CO, HC and NO.sub.x a Pd-catalyst, a Pt-catalyst, a Pd-Pt binary catalyst and a Pt-Rh binary catalyst have been widely in use.
Among the four items mentioned above, except for the catalyst constituted of Pd only or constituted of Pt-Pd with Pd as main element, the catalyst mainly constituted of Pt as catalytic element shows an excellent purifying performance, but its performance is poor in the range of low exhaust gas temperatures such as at the time of engine start or idling.
The present inventors have improved the purifying performance in the low temperature range of the catalyst mainly constituted of Pt as catalytic element and succeeded in realizing an exhaust gas purifying catalyst exhibiting an excellent performance over a wide range of temperatures.